Wife leaving when cot light's off? Sleeper may want these on (12)
I believe the answer is:
nightclothes
'sleeper may want these on' is the definition.
(nightclothes are worn when sleeping)
'wife leaving when cot light's off?' is the wordplay.
'wife' becomes 'w'.
'leaving' indicates named letters should be taken away.
'off?' indicates an anagram (I've seen 'off' mean this).
'when'+'cot'+'lights'='whencotlights'
'whencotlights' with 'w' taken away is 'hencotlights'.
'hencotlights' anagrammed gives 'NIGHTCLOTHES'.
(Other definitions for nightclothes that I've seen before include "sleepers' get-up" , "Items worn in bed" .)